CONTENTS
Electron arc therapy.
Introduction to electron arc therapy
Calibration of electron arc therapy
field shaping
beam energy
Treatment planning
location of the isocentre
scanning field width
collimation used in electron arc therapy.
summary
2. Introduction
• Electrons interact with atoms by different processes
through the Coulomb force.
• These processes are,
1. Inelastic collisions with atomic electrons
(ionization/excitation);
2. Inelastic collisions with nuclei (bremsstrahlung);
3. Elastic collisions with atomic electrons;
4. Elastic collisions with nuclei (no energy loss, large
angle deflection).
3. Intro…
• In inelastic collisions, some of the kinetic energy is lost in
producing ionization or converted to other forms of energy.
• In elastic collisions, kinetic energy is not lost but it may be
redistributed among the emerging particles.
• In low-Z media (water, tissue), electrons lose energy through
ionization and excitation.
• In high- Z media (tungsten, lead), bremsstrahlung is important.
• In ionization, if the ejected electron is energetic enough to cause
further ionization, it is called secondary electron or d-ray.
• (note: by definition, the energy of the d-ray is < ½ of the incident
electron energy)
• Electrons continuously lose its energy traveling through the
medium.
4. Electron Arc Therapy
• An electron beam arc therapy technique has been developed
for the treatment of the post-mastectomy chest wall using a
clinical linear accelerator modified for arc therapy.
• Problems encountered using stationary photon or electron
fields to irradiate large chest wall areas include:
i. Excessive treatment of a sizable volume of normal tissue in
order to encompass extended treatment portals on a curved
surface.
ii. Inhomogeneities of radiation dose at field abutments; and
iii. Variations of radiation dose throughout the treatment
volume secondary to irregular patient contours.
5. Electron Arc Therapy:
• Electron beam arc technique gives excellent dose distribution
for treating superficial tumors along curved surfaces.
• On the basis of isodose distribution, electron arc therapy is
most suited for treating superficial volumes that follow curved
surfaces such as the chest wall, ribs, and entire limbs.
• Although all chest wall irradiations can be done with electron
arcing, this technique is mostly useful in cases for which the
tumor involves a large chest wall span and extends posteriorly
beyond the midaxillary line.
6. • The conventional technique of
using tangential photon beams in
this case will irradiate too much of
the underlying lung.
• The alternative approach of using
multiple abutting electron fields is
fraught with field junction
problems, especially when angled
beams are used.
• In short, it appears that for a
certain class of cases, electron arc
therapy has no reasonable
alternative.
Electron Arc Therapy:
7. Calibration of Arc therapy
• Calibration of an electron arc therapy procedure requires
special considerations in addition to those required for
stationary beam treatments.
• Dose per arc can be determined in two ways:
(a) Integration of the stationary beam profiles and
(b) Direct measurement.
8. • (a)Integration of the stationary beam
profiles
• This method requires an isodose
distribution as well as the dose rate
calibration of the field (under stationary
beam conditions) used for arcing.
• Radii are drawn from the isocenter at a
fixed angular interval Δθ (e.g.,10 degrees).
• The isodose chart is placed along each
radius, while the dose at point P as a
fraction of the maximum dose on the
central axis is recorded.
• Let D,(P) be this dose as the isodose chart
is placed at the radius.
9. • The dose per arc at P is given by the following equation:
where
• D0 is the dose rate per minute in the stationary field at the
depth of dmax
• n is the speed of rotation (number of revolutions per minute), and
• lnv (i) is the inverse square law correction for an air gap
between the dotted circle and the beam entry point.
10. • Term , Can be calculated from the graph plotted
against Di (P) vs Radius i.
11. Direct measurement
• It requires a cylindrical phantom of a suitable material such as
polystyrene or Lucite.
• A hole is drilled in the phantom to accommodate the chamber
at a depth corresponding to the dmax·
• The radius of the phantom need only be approximately equal
to the radius of curvature of the patient, because only a small
part of the arc contributes dose to the chamber reading .
• The depth of isocenter must be the same as used for the
treatment. The integrated reading per arc can be converted to
dose per arc by using correction factors normally applicable to
a stationary beam.
12. Treatment planning :
• The treatment planning for electron arc therapy includes
a) Choice of beam energy,
b) Choice of field size,
c) Choice of isocenter,
d) Field shaping, and
e) Isodose distribution.
13. • Beam energy:
• The central axis dose distribution is altered due to field
motion. For a small scanning field width, the depth dose
curve shifts slightly and the beam appears to penetrate
somewhat farther than for a stationary beam.
14. • The surface dose is reduced and the bremsstrahlung dose at
the isocenter is increased. This phenomenon is known as the
"velocity effect": A deeper point is exposed to the beam
longer than a shallower point, resulting in apparent
enhancement of beam penetration.
‘velocity effect’
15. • Scanning Field Width
• Although any field width may be used to produce acceptable
isodose distribution, smaller scanning fields(e.g., width of 5
cm or less)give lower dose rate and greater x-ray
contamination.
• However, small field widths allow almost normal incidence of
the beam on the surface, thus simplifying dosimetry.
• Another advantage of the smaller field width is that the dose
per arc is less dependent on the total arc angle.
• For these reasons, a geometric field width of 4 to 8cm at the
isocenter is recommended for most clinical situations.
16. Location of isocenter
• The isocenter should be placed at a point approximately
equidistant from the surface contour for all beam angles.
• In addition, the depth of isocenter must be greater than the
maximum range of electrons so that there is no accumulation
of electron dose at the isocenter.
Field Shaping
• Without electron collimation at the patient surface, the dose
falloff at the treatment field borders is rather gradual.
• To sharpen the distribution, lead strips or cutouts should be
used to define the arc limits as well as the field limits in the
length direction cast shielding has been found to be useful for
routine electron arc therapy.
17. • Field Shaping
Isodose distribution in arc rotation with and without lead strips at the ends of the
arc, using a section of an Alderson Rando phantom closely simulating an actual
patient cross section.
Arc angle = 236 degrees;
Average radius of curvature =10cm;
Beam energy = 10 MeV;
lead strip thickness = 6 mm;
Field size at the surface = 4.2 x 8.5cm2.
18. Collimation in Electron arc therapy
• Collimation plays an important role in Electron arc therapy.
• Collimation in electron arc therapy includes,
1. Primary collimation
2. Secondary collimation
3. Tertiary collimation
• Primary collimation: Collimation provided by the Internal
jaws of the collimator (X & Y).
• Secondary collimation: Collimation provided by the External
electron cone applied to attenuate the penumbral beam.
• Tertiary collimation: Collimation by the Lead strips placed on
the patient body during the treatment.
19. Tertiary collimation(Using lead)
• Lead strips are recommended to use for electron arc therapy
treatment due to materials high density (11.4 gm/cm2).
• Lead shielding attenuates the dose fall of (low energy electron
isodose curves) at the ends of the treating contours.
• Electron energy used for the treatment is based on calculation
of treatment thickness to be treated (eg., if 3 cm of tissue is to
be treated 9Mev energy is used for the treatment or one third
of energy )
• Lead strips of 1mm for each 2Mev is recommended to use
during the treatment (eg., for 6Mev electron beam energy
3mm of lead shielding strips are recommended).
• Additional thickness of 1mm of lead shield is recommended
to be added as safety P.O.V
20. summary
• Electron arc therapy can be used as a valuable technique for
treatment of extended superficial fields such as post-
mastectomy chest walls.
• Electron arc therapy offers advantages in dose uniformity and
reduction in dose to underlying lung, when compared to
standard treatment techniques.
• Optimization techniques and variable rad/degree offer further
improvement in dose distributions.
• If consideration is given to the factors influencing dose
distribution in arc therapy, good agreement can be achieved
between computerized treatment plans and the dose actually
delivered.
21. • Since the dose distributions resulting from electron arc
therapy are strongly dependent upon the size, shape and
construction of the primary, secondary, and tertiary
collimators, thorough dosimetric studies of the collimation
system to be used on a specific clinical accelerator should be
completed before treatment using this technique is instituted.
22. Drawback of Electron arc therapy
• After the mastectomy the irregularity of the surface of the
patient body contour increases.
• The anatomical structure of body during the treatment
changes rapidly and doesn't provides a good support for
electron arc therapy
• It is not regular scene to get the body contour of the patient
to be cylindrical accurately.
• Hence the electron arc therapy is not in regular practice in
many institutions.
23. Field gap in photon therapy(arc)
• Adjacent fields using the photons in the arc therapy produces
the hotspot inside the body contour.
• As the isodose produced due the adjacent fields meet inside
at a certain depth inside the body contour.
• To avoid this problem of delivering the hotspot inside the
body certain Gap should be provided between the adjacent
body field-sizes.
• This will avoid the production of the hotspot inside the body
of the patient after certain depth.
• The gap to be given can be calculated by the similar triangle
formula.
25. Calculation
• From similar triangle formula
• ED=2cm,AC=100cm, BC=12.5cm
• Calculated EB =0.25 cm
• Similarly,
• EF = 0.30cm
The calculated gap FB = 0.25+0.30= 0.55cm
BC
EB
AC
ED
26. Vinay Desai
M.Sc Radiation Physics
Radiation Physics Department
KIDWAI MEMORIAL INSTITUTE OF ONCOLOGY
Bengaluru
Thank you… E-mail:- vinaydesaimsc@gmail.com